May 19, 1898J 



NATURE 



65 



of the behaviour of those cells in which no centrosomes have ' 

 been discovered in spite of infinite toil having; been spent on I 

 the attempt to prove their existence, it seems more probable 

 that they are not to be regarded as morphological structures i 

 ranking with nuclei or plastids, but at most as consisting of ; 

 matter which may be condensed to a granular form, or which 

 may be present or be manufactured in a state diffused through 

 the protoplasm. Indeed this matter may perhaps not be in- j 

 appropriately compared with zymogens, which, when suitably , 

 acted upon, liberate substances capable of exerting an influence | 

 altogether incommensurable with their amount on materials : 

 within the scope of their power. But no one would probably 

 go so far as to elevate a lump of zymogen, if it could be shown 

 to exist in a given cell or tissue, to the rank of a cell org;in, any 

 more than most people regard the elaborated spindle fibies as 

 representing anything but a specialised phase of protoplasmic 

 structure, at most temporarily differentiated from the rest of the 

 cell substance, and destined, sooner or later, to be re-absorbed 

 into it, although the remains of some spindles persist long after 

 the cells in which they were formed {de novo) have completed 

 their division. 



Having briefly glanced at the centrosome, we may pass on to 

 consider some of the more important peculiarities connected 

 with the actual process of division of the nucleus. And, first, 

 we will consider the mode of the formation and division of those 

 remarkable structures — the cliromrisomes. During its resting 

 state, a nucleus presents a granular or spongy appearance, and 

 is commonly seen to contain one or more refractive bodies — the 

 nucleoli. As the stages of approaching division are passed 

 through, a substance (which can be identified also in the resting 

 state), known as the chromatin, begins to assume a growing 

 importance. This substance, which consists largely of nucleic 

 acid, aggregates along more or less definite tracts of the colour- 

 less and less stainable matrix (linin) within the nucleus, and 

 finally nearly all the linin is used to provide a substratum in 

 which the chromatin is embedded. This linin scaffolding as- 

 sumes the appearance of a much convoluted thread or threads, 

 and, owing to the predominance of the chromatin, its existence 

 is easily (and often) overlooked. The thread then shortens and 

 thickens, and eventually breaks transversely into a definite 

 number of segments constant for the particular species. Mean- 

 time the well-known spindle is formed, and the chromosomes 

 become arrayed around it (Figs. 2, 4). They are now seen to 

 split longitudinally, and finally the two halves separate, passing 

 io opposite ends of the spindle, where they help to reconstitute 

 he daughter- nuclei which arise in this way. Now, since the 

 original chromatin containing thread appears to be symmetrical 

 about its long axis, it is clear that there exists no obvious grounds 

 for assuming that the two groups of chromosomes, which have 

 ultimately arisen as the result of a longitudinal fission of this 

 'bread, represent anything but the reflected images of each 

 ther ; and indeed there is a great deal which strongly suggests 

 hat the significance of the complicated stages pas'ed through, 

 lies in the ensuring of a qualitatively equal distribution of 

 material to each of the two daughter cells ; quantitative Q<\\xzX\X.y 

 is also secured far more accurately than would probably be 

 the case if each chromosome divided transversely instead of 

 longitudinally. 



The reappearance of a definite number of chromosomes, as 

 well as a priori considerations, based on the relations which 

 on good grounds believed to obtain between the chromo- 

 somes and the existence of hereditary qualities in an organism, 

 have led many investigators to believe that they are the same 

 chromosomes which constantly reappear at each karyokinetic 

 period ; although, in the majority of instances, they cannot be 

 recognised in the intervening state of rest between the successive 

 divisions. This view is, perhaps, hardly sufficiently warranted 

 by the facts, and some of its warmest supporters have been 

 obliged to take refuge in expressions such as a "physiological 

 persistence" ; a sort of persistence which may be entertained as 

 a pious opinion, but which, when one tries to rigorously define 

 it, proves as elusive as metaphors usually are. 



But the chief interest which centres in the chromosomes 

 depends on the remarkable part played by these bodies in con- 

 nection with the reproductive processes. Since every act of \ 

 fertilisation consists essentially m the union of two cells and of 

 their contained nuclei, it is clear that the resulting nucleus will 

 possess twice as many chromosomes as that in each of the cells 

 which have fused together. And if this is repeated in consecu- 

 tive generations it is obvious that the chromosomes, increasing 



in geometrical progression, will soon become too numerous to 

 be contained wiihin the limits of any one nucleus. Hence the 

 necessity of a reduction in their number at some period between 

 each act of fertilisation. This reduction regularly occurs, and 

 always happens at a definite period in the history of the 

 organism, although the exact epoch may differ considerably in 

 different groups of plants or animals. 



A considerable discussion has arisen as to the exact signi- 

 ficance to be attached to the process, over and above the bare fact 

 of the halving of the number of the chromosomes. Some have 

 tried to show that variation, so characteristic of animals and 

 plants, is ensured by the distribution of entire chromosomes 

 between the two daughter-nuclei ; others have seen in it a 

 return to an "embryonic condition" which renders the act 

 of fertilisation a necessary antecedent to further development ; 

 others, including Strasburger, whilst recognising that it is 

 preparatory to fertilisation, and that it indirectly promotes 

 variation by rendering the fusion with another cell pos- 

 sible, regard it as the expression of a return to an ancestral 

 condition, which prevailed before fertilisation by the union of 

 two individuals had come into existence. Of the explanations 

 here mentioned the first is the most consistent, or at least is, at 

 first sight, less obviously contradicted by facts than the rest. 

 But, nevertheless, it will be seen that it docs not by any means 

 embrace all the well workedout cases, and therefore cannot be 

 considered as of general application. It will, however, be 

 specially considered here, because it is so often brought forward 

 as a most important argument in support of Weismann's theory 

 of Heredity. 



Weismann, as is well known, regarded the hereditary quali- 

 ties of an individual as closely bound up with certain cellular 

 structures, and he has identified these with the minute particles 

 of chromatin which in the aggregate go to form a chromosome. 

 Each chromosome is conceived of as possessing the material 

 substrata for all the specific characters of the organism, but the 

 arrangement or constitution of these is slightly different in the 

 different chromosomes. The actual course of development, 

 followed by the organism as a whole, depends on the degree in 

 which one or other group of characters becomes predominant, 

 or on the result of a compromise between them. 



Clearly, therefore, whilst an organism which had lost half its 

 chromosomes could not be expected to exhibit as many possi- 

 bilities ot variation as one which retained its full number (if 

 development were possible at all under such circumstances), by 

 the elimination of the half, and subsequent replacement of them 

 by corresponding (but slightly differing) chromosomes from 

 another individual, the chancesof new variation would certainly, 

 if we accept the premises, be greatly increased. 



These views have been worked out in great detail, and they 

 have received quite a remarkable confirmation as the result of 

 the researches of Riickert, Hacker, vom Rath, and others. But, 

 whilst recognising the great interest attaching to the results 

 obtained by these investigations, it is at present quite impos- 

 sible to regard them as affording more than a local confirmation 

 of Weismann's theories, simply because, although they may pos- 

 sibly bear this interpretation, there are (as already indicated) 

 other cases which even Procrustes himself could noi fit into the 

 same bed. 



As regards the general character of the "reduction division.s," 

 there naturally exists a certain amount of variety in detail; 

 but in the following summary' an attempt will be made to 

 present the more salient and lundamental features of the pro- 

 cess. If one takes as an example a higher animal, the reduction 

 divisions are seen to be closely related with the formation of the 

 actual sexual cells— ova and spermatozoa ; up to the penultimate 

 divisions the line of cell generations have possessed nuclei with a 

 definite number of chromosomes, which we will designate as 

 2«. Then follows a long period of repose and of growth, and 

 when the nuclei of these cells emerge from their quiescent con- 

 dition, the number of their contained chromosomes is seen not 

 to be 2«, but only n. That is to say that a numerical reduction 

 has, somehow, been accomplished in the resting period. There 

 is no question here of any chromosomes having been eliminated ; 

 nothing has been expelled (.so far as can be seen) from the 

 nucleus, but there has been a rearrangement. It has been sug- 

 gested, and the view is stoutly maintained by Hacker and others, 

 that the reduction here is only apparent, and that what has 

 really occurred is that the original thread has only, so to speak, 

 broken transversely at every other joint, leaving two chromo- 

 .somes attached end to end. Each appaient chromosome then 



NO. 1490, VOL. 58] 



